1. Technical Field
The present invention relates to a technology for automatically adjusting the focus of a lens.
2. Related Art
Latest still cameras and video cameras generally have the auto focusing function. One proposed technology of auto focusing control moves a focus lens in a lens unit to maximize an evaluation value calculated from a high frequency component of a luminance value of a captured image as shown in FIG. 12 (see, for example, Japanese Patent Laid-Open No. H07-87377 and No. H01-125065).
A moving mechanism of moving the lens in the lens unit generally has a stepping motor equipped with a lead screw, a lens frame for supporting the lens, and a guide bar for guiding the motion of the lens frame in the direction of an optical axis. The lens frame has a rack for transmitting the power of the motor to the lens frame. The rack is joined with the lead screw of the motor. The motor is driven to rotate the lead screw. The rack converts the rotational force of the lead screw to the moving force of the lens frame in the direction of the optical axis.
In the lens unit, a backlash of the joint between the lead screw and the lens frame may lead to a variation in moving distance of the lens according to the moving direction of the lens. One proposed technology against this problem disclosed in Japanese Patent Laid-Open No. H08-265620 adds an additional moving distance to the moving distance of the lens on the occasion of inverting the moving direction of the lens, so as to compensate for the variation in moving distance.
On the occasion of inverting the moving direction of the lens, however, there may be a positional deviation of the optical axis of the lens from its center axis or a change in inclination of the lens, in addition to the variation in moving distance of the lens. The proposed technology of the above cited reference has difficulty in dealing with such a potential positional deviation of the optical axis and a potential inclination change of the lens.